Glass fibres for optical communication are made from high purity, silica-based glass fibres drawn from glass preforms, which preforms are produced according to various glass deposition techniques.
Some of these deposition techniques, including vapour axial deposition (VAD) and outside vapour deposition (OVD), are based on flame combustion wherein reactants (i.e. silica precursors, such as SiCl4, optionally together with dopants materials, such as GeCl4, for suitably modifying the refractive index of the glass) are fed together with combusting gases through a deposition burner which directs a high temperature flow of forming fine glass particles onto a rotating growing target preform.
According to the VAD deposition technique, the growth of the preform takes place in an axial direction. Thus, the deposition burner(s) is typically maintained in a substantially fixed position, while the rotating preform is slowly moved upwardly (or downwardly) with respect to the burner, in order to cause the axial growth of the preform. Alternatively, the rotating preform can be maintained in a substantially fixed position, while the deposition burner is slowly moved downwardly (or upwardly) with respect to the preform.
Differently from the VAD technique, in the OVD technique the growth of the preform takes place in a radial direction. In this case, a rotating target (e.g. a quartz glass rod) is generally positioned in a fixed horizontal or vertical position and the deposition burner is repeatedly passed along the surface of the growing preform for causing the radial growth of the same.
Independently from the applied deposition technique, a porous glass preform is thus fabricated, which is then consolidated to form a solid glass preform apt for being subsequently drawn into an optical fibre.
Typically, an optical fibre preform comprises a central portion (core) and an outer portion (cladding), the core and the cladding differing in their respective chemical composition and having thus different refractive indexes. As in the optical fibres, the cladding portion forms the majority of the preform. The preform is typically manufactured by producing and consolidating a first preform comprising the core and a first portion of the cladding. An overcladding layer is then deposited onto said first preform, thus obtaining a porous preform, which is then consolidated into the final preform.
In general, conventional burners for manufacturing optical fibre preforms are made up of a plurality of coaxial tubes through which the glass precursor materials (i.e. silica precursors, such as SiCl4, optionally together with dopants materials, such as GeCl4), the combusting gases (e.g. oxygen and hydrogen or methane) and, optionally, some inert gas (e.g. argon or helium) are fed. Typically, the glass precursor material is fed through the central tube of the burner, while other gases are fed through the annular channels defined by the coaxially disposed tubes.
Generally, the gases are introduced at one extremity of each annular channel.
U.S. Pat. No. 4,417,692 describes a burner for manufacturing an optical fibre preform comprising a plurality of coaxial tubes defining a plurality of annular channels between each pair of adjacent tubes, having an annular chamber at the extremity of each annular channel. The chambers are radially delimited by two cylindrical concentric surfaces. A feeding duct is connected to each chamber to feed a gas into it. The feeding ducts are disposed perpendicularly to the axis of the coaxial tubes; their direction thus intersects the inner surface delimiting the annular chambers.
U.S. Pat. No. 4,661,140 describes a burner for manufacturing an optical fibre preform comprising a plurality of coaxial tubes defining a plurality of annular channels between each pair of adjacent tubes. The gas is fed into the annular channels directly by means of pipes disposed perpendicularly to the axis of the coaxial tubes. Also in this case, the direction of said pipes intersects the inner surface delimiting said annular channels.
The Applicant has however observed that the disposition, of said feeding ducts or pipes may not allow a completely satisfactory uniform distribution of the gas flowing through the annular channels of the burners. For possibly optimising the gas distribution along the circumference of said channels, the Applicant has now found a new method for feeding the gases into distribution chambers that are connected to said annular channels and a burner for implementing the said method. It has in fact been found that by imparting to the flow of gas fed to the distribution chambers a direction not incident to the axis of the coaxial tubes, in particular a direction substantially tangential to the inner surface of the distribution chamber the problem can be overcome.
Advantageously, an optimised distribution of gases according to the present invention may allow using deposition burners having shorter lengths. As a matter of fact, in the burners of the prior art, a substantial length of the tubes forming the annular channels is necessary, in order to allow the flow of gas to reach a substantial uniformity before exiting from said annular channels. According to the present invention, a substantially uniform flow of gas is instead obtained within a relatively short distance from the entrance of said gas into the annular channels. Consequently, the length of the tubes forming the annular channels can be advantageously reduced, if desired.